Traditionally hearing aid fitting is done by measuring audiogram by test tones of different frequencies and amplitude. In case the user needs a bone conductor hearing aid, an additional audiogram showing the bone-conducting-loss is to be measured. Both types of audiograms are measured in frequency steps of 5 dB's or 10 dB's. For air-conductive hearing aids, the typical sound tolerance spread between devices is about +/−3 dB at 1 kHz and +/−6 dB at frequencies below 500 Hz and above 2000 Hz.
Sometimes a “real-ear measurement” is performed to check if the client receives the prescribed gain. In this type of measurement, the precision of the results depends on the correct placement of a measurement tube, for a calibrated microphone, in the ear canal—together with the ITE-HA or ear mould of the BTE hearing aid. This measurement will give a good idea of the sound pressure, but may never be fully correct because of the extra leaky venting caused by the extra tube arranged in the ear.
All in all this leaves tolerances both on the supplied hearing aids and on the measurement type which means that the measures will never be as precise as wished. This is usually handled by a further subjective trimming, which is based on feed back on different listening situations obtained from the client.
The precision of the above fitting methods is based on measurement of the client's hearing threshold on traditionally calibrated equipment—but equipment which is not normally worn by the client.
Partially to handle this, it is known to use an internal tone generator in some hearing aids, by way of which an on-the-ear-audiogram can be obtained. This method of measuring the hearing threshold will be more precise with regards to compensating for the tolerances between devices. And also no venting issue is caused by the use of an audiometer microphone tube.
It is well known that performing bone-conduction (BC) audiometry is prone to large tolerances on the obtained results. The major reason for this is that traditional BC-audiometry is based on a measurement method which has built-in large inaccuracies.
Most of these measurements are done using a calibrated vibrator which is pressed to the skin and thus skull-bone—usually behind the ear. By providing well defined sinus tones of different levels and frequencies from the vibrator, the hearing threshold and/or uncomfortable level (UCL) can be measured by tracking reaction from the patient. However, this method is prone to inaccuracies due to the uneven attenuation of the skin—depending of pressure applied to the vibrator, and due to the slight frequency dependency on the transfer function of the skin, and also the ability to place the vibrators point of attack at exactly the same place each time. This also may be depending on which audiologist are attaching the vibrator to the head of the patient.
All in all, it is known that the total sum of these inaccuracies, in terms of repeatability, is in the range of 15-20 dB or even worse. This may cause patients with BC-losses to unintentionally get their hearing aids incorrectly fitted.
According to the invention it is attempted to assist in getting the best audiological fitting in order to help the patients to obtain an ossointegrated implant, which transfers the vibration from the BC hearing aid to the skull and thus to the cochlea in a way which best account for the hearing deficit of the user.
The idea is based on the fact that the fixed connection through the skin comprising implant and skin penetrating abutment is avoiding the uneven and placement-dependent attenuation of the skin and its texture. Also this provides the exact same point of vibration each time the BC-hearing aid is used. Thus a vibration applied to the implant will have same transfer-function of vibration to the cochlea each time used.
At the same time, bone-conductive hearing device may have a built-in sine generator which can be adjusted to all audiological relevant frequencies such as in the range from 200 Hz-10 kHz and precisely adjustable amplitude.
By combining this with a vibrator which has fairly small gain tolerances, well defined combinations of frequencies and amplitudes can be provided. This is very similar to a traditional BC-conductive audiometer but with expected applied amplitude accuracies about +/−2-3 dB. This means that an audiometric test can be done “on the patient” with more precise accuracy than earlier.
It is also known to use a calibrated vibrator in connection with a traditional audiometer. But such a calibrated vibrator is not the same as the one which the hearing aid user will finally wear after the fitting.